Generally, the input selection circuit adopted in the selector of a composite electronic appliance such as a multiplayer is constituted as shown in FIG. 1, and is operated as described below. Upon supplying of control signals CA, CB from a microcomputer (not shown) as a controller to a switching controller 1 of the input selection circuit, the switching controller 1 decodes two control signals and selects a desired analogue switch from a plurality of bidirectional switches which are provided in a bidirectional switch device 2.
Thus, if a bidirectional analogue switch is selected by decoded control signals, then an analogue output signal of the signal source coupled to the selected bidirectional analogue switch among a plurality of signal sources VS1-VSn, will be outputted through an amplifier 3.
The above procedure of operation will be described in more detail by referring to FIG. 2 where an equivalent circuit for the circuit of FIG. 1 is illustrated. In FIG. 2, the one side terminals x1-xn of a plurality of bidirectional switches SW1-SWn provided in the bidirectional switch device 2 will be indicated by X, and the other side terminals y1-yn thereof will be indicated by Y. When the current outputted from the signal sources VSL-VSn flows in a direction from X to Y of the bidirectional switches SW1-SWn, the on-current resistance of the switches SW1-SWn will be represented by R(Y/X), and on the other hand, when the current flows from Y to X, the on-current resistance will be indicated by R(Y/X). Then a case of R(Y/X).noteq.R(Y/X) will occur due to the existence of a directional conduction deviation generated by the internal resistances IR of the switches SW1-SWn. Therefore, the input/output voltage characteristics will become non-linear, i.e. T1.noteq.T2, as shown in FIG. 3 due to the existence of the directional conduction deviation of the switches SW1-SWn, thereby causing a distortion during the transmission of signals. The signal distortion due to the existence of the directional conduction deviation of the switches SW1-SWn can be calculated by the following formulas which are based on the feedback theory. ##EQU1## where T(S) represents the total transfer function,
G(S) the transfer function, and PA1 H(S) the feedback function.
Also when G(S).multidot.H(S)&gt;&gt;1, T(S) will be given as follows; ##EQU2## Based on the above formulas, if it is assumed that the current flows in a direction from X to Y of the switches SW1-SWn, then the following formula is obtained; ##EQU3## On the other hand, if it is assumed that the current flows from Y to X of the switches SW1-SWn, then the following formula is obtained; ##EQU4## where RS represents RS1, RS2 . . . or RSn, and RI represents R1, R2 . . . or Rn. Meanwhile, the sensitivity function ##EQU5## which represents the ratio of the variation rate of T(S) to the variation rate of H(S), ##EQU6## can be given as follows: ##EQU7## As is apparent from the above formula (3), the closed loop gain of the bidirectional switches SW1-SWn against the variation of H(S) is very large. Therefore, if a sinusoidal wave input signal originated from the ground potential is fed from the signal sources VS1-VSn to the bidirectional switches SW1-SWn, then there arises a problem that the signal outputted from the amplifier 3 of FIG. 2 becomes unsymmetrical.